When natural gas is extracted from a subterranean formation it flows to the earth's surface and is collected at the well site. Natural gas contains essentially hydrocarbons, but invariably includes entrained water that is usually in the form of water vapor. The raw gas can also include, depending upon the nature of the underground reservoir, pollutants such as hydrogen sulfide (H2S), volatile organic compounds (VOCs), and other contaminants such as BTEX (benzene, toluene, ethylbenzene and xylenes).
Entrained water is a problem to the transportation, storage and use of natural gas, as it readily condenses into liquid when cooler temperatures and decreased vapor pressure are encountered at the earth's surface. The entrained water can cause problems in pipeline and process equipment including corrosion, and collects in low places in a pipeline where it can freeze into an ice with cold temperatures, to a point that the flow through a line can be severely restricted or blocked. Accordingly, in the oil and gas industry it is customary to extract as much of the entrained water as possible before the natural gas is passed to a pipeline for transportation to an area for storage or use.
The most common means employed in the petroleum industry to extract water from natural gas is by the use of liquid dehydrators. In this process the natural gas is conducted into a vessel, commonly known as a contacting tower or scrubber, in which it is intimately mixed with a liquid desiccant such as glycol. Glycol makes an ideal liquid desiccant for natural gas because it is relatively inexpensive, has a relatively high boiling point, does not easily oxidize and is recyclable. When the natural gas contacts the glycol, the entrained water or water vapor carried in the natural gas is absorbed by the glycol. The dehydrated or “dry” natural gas can then separated from the glycol and passed to a pipeline for storage or use.
Meanwhile, the glycol (referred to as “wet glycol”), is conducted to a separate vessel, commonly known as a reboiler or reconcentrator, where the wet glycol is heated to a temperature above the boiling point of water but below the boiling point of the glycol, allowing the glycol to remain in a liquid state while the water is boiled off and converted to a vapor state. The “dry glycol” can then be cycled back to the scrubber for the treatment of additional natural gas.
In the past, the vapor that was created in the reboiler was simply vented to the atmosphere. If the vapor is one-hundred percent water, that is pure water, the venting of the water vapor to the atmosphere is not harmful to the environment. Inevitably, however, the vapor passing from a glycol reboiler includes other contaminants and pollutants, particularly BTEX and VOCs, and venting these contaminants to the atmosphere is becoming an increasing environmental problem. Environmental laws have been enacted in recent years that mandate that the discharge of these pollutants to the atmosphere should be substantially reduced, if not eliminated.